Conventionally, such outlets--usually provided at the bottom of the container--are closable by a slider having a cutout which conforms to the outlet opening and is alignable therewith in an unloading position, the outlet being obstructed by a solid portion of the slider upon displacement thereof to a closure position. The slider is sandwiched between upper and lower gaskets respectively carried on the rim of the outlet and on a downward extension thereof, these gaskets being intended to insure an airtight seal when the outlet is closed. In the closure position, the weight of the stored goods bears upon the slider with resulting compression of the lower gasket; this may lead to a separation of the slider from the upper gasket, with particles of the bulk material penetrating into the resulting gaps. The subsequent shifting of the slider causes these particles to rub against the upper gasket which is therefore subjected to considerable wear. Thus, the desired airtight seal will no longer be maintainable.